Can't mount /data or /storage from TWRP on Surnia (Moto E).

This is causing problems because it means I can't back-up, wipe or - presumably - install a ROM. I read about changing the type of file-system but I don't think that's something I want to do before I have a back-up. Has anyone got any ideas?

The version of TWRP I'm using is twrp-3.0.2-0-surnia.img